Many types of surfaces require cleaning and decontamination of coatings and residues without significant impact to the base surface. It is desirable to aggressively clean a variety of coatings and contaminants without leaving behind additional cleaning residues, such as chemical solvents, water, grit media, etc. This is particularly problematic in the field of nuclear radioactive facility clean-out and decontamination, as any cleaning substance will likewise become radiologically contaminated. Disposing of large volumes of cleaning materials becomes costly, dangerous, and time consuming. What is therefore desired is a cleaning media imparting high kinetic momentum transfer to relatively hard particles which impact the surface to be cleaned, but then sublimate into a harmless gas. This is particularly important in the cleaning and decontamination of nuclear radioactive related facilities, where even tiny amounts of residual nuclear contamination deposited on surfaces or diffused therein are highly hazardous and expensive to remove and dispose of with conventional methods. As an example, disposal of a single gallon of nuclear radioactive contaminated water used as a cleaning agent can cost in excess of $1000. To dispose of contaminated solid material can cost $50–500 per pound, depending on the contamination level. It is therefore desirable to clean every nook and cranny on equipment and facilities, so that the dismantled structures can be classified as low level waste, which can be cheaply handled and buried at approved nuclear burial sites.
A known method for cleaning involves the use of CO2 pellets accelerated by a source of compressed air. Patents describing the use of CO2 pellets for cleaning include U.S. Pat. No. 5,109,636 to Lloyd, et al. and U.S. Pat. No. 5,445,553 to Cryer, et. al. Other cleaning systems generate a source of CO2 snow, which are, in effect, small diameter solid particles. Cleaning systems generating CO2 snow are described, for example, in U.S. Pat. No. 5,514,024 and U.S. Pat. No. 5,390,450 to Goenka. Nevertheless, the systems described in the referenced patents do not possess sufficient energy to ablate and clean the types of surfaces commonly found in a contaminated nuclear facility. In a nuclear facility, it is desirable to clean painted metals down to the base material, or abrade concrete with up to 2–4 mm surface material removal, because radiological contaminates can directly and indirectly diffuse into porous structures.
Other existing methods of cleaning involve the use of high pressure cryogenic liquids that are sprayed from a high pressure nozzle. U.S. Pat. No. 5,733,174 to Bingham et al., is typical of the use of high pressure cryogenic liquid use. Bingham et al. discloses a slurry of high pressure Nitrogen and CO2 co-existing as a slurry, which is pumped at high pressure and delivered to a surface to be cleaned as a jet. The N2 and CO2 are in a liquid state, the N2 comprising a cryogenic fluid and the CO2 comprising a non-cryogenic fluid. As the N2 and CO2 expand through a high pressure orifice, a phase change occurs. The CO2 is super-chilled and precipitates to solid CO2 particles at high velocity. The solid CO2 particles eventually evaporate, leaving no secondary waste. The disadvantages of such typical cryogenic systems include the required use of rigid, non-flexible high pressure metallic tubing for delivery of the cryogen to the nozzle orifice. Rigid tubing poses severe limitations on the ability to maneuver an orifice cleaning head to desired orientations needed to access complex equipment needing cleaning and decontamination, particularly when such equipment is in highly hazardous closed cells and only robotic access is possible. In addition, rigid cryogenic tubing requires highly effective insulation, since the cryogenic liquid within the tubing is at a very low temperature, and must be maintained at low temperatures until it exits the orifice. Moreover, cryogenic N2 is a very expensive to purchase, deliver, and pump.
Accordingly, there is a need for an improved non-cryogenic cleaning system that can be deployed in remote and inaccessible environments using an ambient temperature low cost flexible hose, and which is much more aggressive in terms of effective material removal.